PM with your email addresses, I'll invite.
The way it apparently works right now is that you don't get an email when you get invited, but after you've been invited, you can just go to plus.google.com and register.

Edited:

I'll add this in here so I don't have to say it every time:
After you've been invited, you will not get any email about it. At least I didn't get one when I was invited.
You have to keep stalking plus.google.com after you've been invited and either register straight up, or if they're at full capacity, wait until they add more capacity.